Zebra Mussels Hard to Find in the St. Croix

National Park Service scientists say the invasive species’ numbers were down again in 2013.




3 minute read

Zebra mussels
Zebra mussels (Photo courtesy U.S. Geological Survey)

A new report from the National Park Service says populations of invasive zebra mussels remained low in 2013, for the third consecutive year. Biologists suspect the decline is due to two primary factors: common carp and high water.

The 2013 study (PDF) was hampered by budget cuts and the federal government shutdown, meaning no SCUBA diving by government biologists starting in July, forcing them to use different methods than previous years to look for the mussels. Different methods can cause different results, the report warns, while striking an optimistic tone.

“Though the data collection in 2013 was less than optimal or expected and is not readily comparable to previous years, the evidence suggest zebra mussel numbers overall in Lake St. Croix are lower than the peak years of the mid-2000s,” the report states.

The non-native zebra mussels first appeared in the St. Croix in 2000. The fingernail-sized creatures can appear in huge numbers, clogging water intakes and boat engines, and damaging docks and other structures in the water.  Their sharp shells can also slice up bare feet of swimmers, and they hurt native species, disrupting the food chain and even hurting fish populations.

O Mussel, Where Art Thou?

Theories about the reason for the zebra mussels’ decline range from invasive Asian carp eating them to warming waters (neither of which are thought to be the cause).

There aren’t any conclusive answers yet, but the report points to two likely factors.

The first is that unusually high water lingering into mid-summer the last few years is washing young zebra mussels downstream before they find something to hold on to. While the St. Croix’s usual pattern is high water in the spring, followed by lower summer flows with a slight rise in the fall, that pattern has not occurred since 2009.

The past few years have seen something new and different, as described the National Park Service report: “persistent high water lasting well into the summer (July), often followed by a complete lack of rainfall for weeks—even lasting into the fall.”

Young zebra mussels take about three weeks to develop before they can attach to the bottom or other structures. The Park Service report states that during a dry year, water stays in one of the key Lake St. Croix pools for 19 days. But during a wet year, it’s just 8 days. The river could simply be carrying them away.

Then, just as water flows have slowed down in July recently, it seems common carp might be moving in to feast on the mussels. The report states, “Gut contents of carp collected during the pilot study in 2007-08, were packed with shells, apparently to the exclusion of other food.”

It’s possible that the carp prefer a specific size of mussel, which is slowing reproduction. The U.S. Geological Survey has been conducting a larger study of this phenomenon, and results are expected to be published this year.

Read the full 2013 National Park Service study for yourself here.

Fighting the good fight

The only way Zebra mussels can move upstream or across land is with help from humans.

While the effort to eradicate the harmful little clams isn’t over in the St. Croix yet, every boater and river user can help ensure we don’t lose the ground we’ve gained recently.

To help, please adopt the following practices:

  • Do not travel upstream past the High Bridge Checkpoint at mile 28.5 north of Stillwater.
  • Remove all aquatic plants and animals from your boat, trailer and accessory equipment (anchors, centerboards, trailer hitch, wheels, rollers, cables and axles) before leaving the boat ramp or marina.
  • Empty your bait bucket on land. Never dump live fish or water into another body of water.
  • Drain all bilge water, live wells and engine compartments.
  • Wash your boat, tackle, downriggers, lines and trailer with hot water (140 degrees).
  • Flush hot water through your motor’s cooling system and other boat parts that normally get wet. Let everything dry for seven days in the sun before transporting your boat to another body of water.
  • Learn more



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Zebra Mussels Hard to Find in the St. Croix